The present invention relates to a receiving apparatus with diversity, which has a reception antenna switching function and is used in the field of mobile communication and, more particularly, to a controller which selectively switches the reception outputs from two antennas and is used in a receiving apparatus for receiving, e.g., FM or TV broadcasting in the VHF or higher frequency band.
Receiving apparatuses with diversity are of two-tuner diversity which switches two sets of reception antennas and receiving apparatuses and antenna switching diversity which switches two reception antennas.
The antenna switching diversity basically operates as follows. Upon detecting from a multi-path distortion (noise) or the like that a multi-path wave (synthesized wave of radio waves which had arrived via different paths) or antenna noise has arrived at one antenna which is currently connected to the tuner unit of the receiving apparatus, the connection to the tuner unit is switched to the other antenna.
An automobile receiving apparatus as prior art 1 is a system using a rod antenna as a main antenna and a glass antenna as a subantenna arranged on the rear windshield of an automobile. When the electric field of a received wave is weak, the antenna connection of the receiving apparatus is forcibly fixed to the rod antenna side. The rod antenna rarely has gain directivity, so the reception sensitivity of the tuner does not depend on the arrival direction of a radio wave.
An automobile receiving apparatus as prior art 2 is a system using two glass antennas which are arranged on the rear windshield of an automobile such that they have orthogonal electrical phase angles (directivities). When the electric field of a received wave is weak, antenna connection of the receiving apparatus is forcibly fixed to one glass antenna side.
This system using two glass antennas is expected to be a future mainstream because it does not impair the outer appearance or running performance of the automobile. More specifically, when connection is forcibly fixed to the rod antenna side for a weak field, as in prior art 1, damage of the rod antenna itself or a mechanical change over time poses a problem. However, in prior art 2, the diversity operation is realized while keeping the glass antenna connected, so the glass antenna itself is hardly damaged.
The arrangement of the conventional receiving apparatus will be briefly described below with reference to FIG. 1. Referring to FIG. 1, an antenna switching circuit 50 selectively connects two glass antennas 51 and 52 to a front end section (high frequency amplification/frequency conversion stage) 53 of the tuner unit.
Reference numeral 54 denotes an intermediate frequency amplifier for limiter-amplifying the output signal (intermediate frequency) from the front end section 53; 55, an FM demodulator for demodulating an FM signal from the output signal from the intermediate frequency amplifier 54; 56, a voltage-controlled amplifier with a soft muting function, which amplifies the output signal from the FM demodulator 55; and 57, a stereodecoder for decoding right and left component signals from the output signal (FM stereo composite signal) from the voltage-controlled amplifier 56.
A field strength detector (FSD) 58 detects the level of the output signal (intermediate frequency) from the intermediate frequency amplifier 54 to detect the field strength of the received radio wave.
A gain controller 59 controls the amplification gain of the voltage-controlled amplifier 56 on the basis of the output signal (signal meter output) from the FSD 58.
An antenna diversity controller 60 controls the antenna switching circuit 50 on the basis of the output signal from the FSD 58.
FIG. 2 is a block diagram showing a conventional example of the antenna diversity controller 60 shown in FIG. 1.
Referring to FIG. 2, an average variation detector 61 is constituted by, e.g., a CR circuit to weight the output (signal meter output) from the FSD 58 with a certain time constant and detect the average level variation.
An AC boost circuit 62 extracts the AC component of the output signal (signal meter output) from the FSD 58 and boosts it. The AC boost circuit 62 is arranged to determine the diversity operation sensitivity.
Reference numeral 63 denotes a bottom peak detector (BPD) for detecting the bottom peak of the output signal from the AC boost circuit 62; 64, a voltage comparator for comparing the output level from the average variation detector 61 with that from the BPD 63; and 65, a reset circuit for generating, on the basis of the output from the voltage comparator 64, a reset signal for resetting the BPD 63.
A flip-flop circuit 66 latches the output from the voltage comparator 64. The set output signal from the flip-flop circuit 66 is used to control the antenna switching circuit 50 to select one main antenna 51. The set output signal is inverted by an inverter 67. The inverted signal is used to control the antenna switching circuit 50 to select the other subantenna 52.
Since a glass antenna generally has a high gain directivity of about 20 dB in both the horizontal and vertical directions, the reception sensitivity of the receiving apparatus depends on the arrival direction of a radio wave.
When two glass antennas are arranged such that the directivities are orthogonalized, as described above, the reception sensitivity of the receiving apparatus abruptly lowers (drops to the null point) for a wave which has arrived at an angle of about .pi./4 (=45.degree.) with respect to the electrical phase angle of the two glass antennas, and the antenna connection switching condition cannot be obtained.
More specifically, even when a radio wave having a field strength of, e.g., a bit higher than 20 dB, which can hardly be low, has arrived at the currently connected glass antenna at the angle of about 45.degree., the electric field is regarded as weak, and antenna connection of the receiving apparatus is switched to a specific glass antenna side.
However, if the condition is so bad that the radio wave cannot arrive at this glass antenna, neither multi-path distortion nor antenna noise can be detected because of very poor reception quality. Therefore, even when the radio wave is arriving at the other glass antenna, the antenna connection switching condition cannot be obtained, resulting in a fatal defect.
In addition, when the strength of the received field fairly lowers, as described above, soft muting (function of moderately muting the audio signal in response to the phenomenon of the received field to suppress an increase in white noise unique to the receiving apparatus) which is normally employed in the audio signal processing stage of the receiving apparatus suddenly functions. Consequently, muting is suddenly performed to largely impede the auditory sense.
As described above, the conventional receiving apparatus with diversity cannot detect a multi-path wave or antenna noise unless the radio wave arrives at the currently connected antenna. For this reason, even when the radio wave is arriving at the other antenna, the antenna connection switching condition cannot be obtained, resulting in a fatal defect.